2,180 research outputs found
Two photon decay of the pseudoscalars, the chiral symmetry breaking corrections
The extrapolation of the decay amplitudes of the pseudoscalar mesons into two
photons from the soft meson limit where it is obtained from the axial-anomaly
to the mass shell involves the contribution of the 0^ - continuum. These chiral
symmetry breaking corrections turn out to be large. The effects of these
corrections on the calculated pi ^0 decay rate, on the values of the
singlet-octet mixing angle and on the ratios f_8 / f_pi and f_0 / f_pi are
discussed. The implications for the transition form factors gamma gamma* -->
pseudoscalars are also evaluated and confronted with the available experimental
data.Comment: Computational errors straightened and study of the implications on
the form factors of the transitions gamma gamma* --> pseudoscalars adde
On the NLO Power Correction to Photon-Pion Transition Form Factor
We propose a perturbative evaluation for the next-to-leading-order (NLO)
power correction to the photon-pion transition form factor. The
effects of the NLO power correction are analyzed.Comment: 4 pages, 3 figures, Revtex, revised versio
Detecting and Studying Higgs Bosons at a Photon-Photon Collider
We examine the potential for detecting and studying Higgs bosons at a
photon-photon collider facility associated with a future linear collider. Our
study incorporates realistic \gam\gam luminosity spectra based on the most
probable available laser technology. Results include detector simulations. We
study the cases of: a) a SM-like Higgs boson; b) the heavy MSSM Higgs bosons;
c) a Higgs boson with no couplings from a general two Higgs doublet
model.Comment: 52 pages, 26 figures, revised version with new appendi
Quark-hadron duality, axial anomaly and mixing
Interplay between axial anomaly and quark-hadron duality in the presence of
strong mixing is considered. The anomaly sum rule for meson transition form
factors based on the dispersive representation of axial anomaly and
quark-hadron duality in octet channel is analyzed. The comparison of this sum
rule to the experimental data on and mesons transition form
factors shows that the interval of duality in this channel is rather small,
contradicting the usual understanding of quark-hadron duality. The same values
of interval of duality are supported by considering the two-point correlator in
the local duality limit. This contradiction may be resolved by introducing of
some nonperturbative non-OPE correction to the relevant spectral density. The
form and value of this correction are discussed.Comment: 9 pages, 1 figure, reference adde
Instanton Contribution to the Pion Electro-Magnetic Formfactor at Q^2 > 1 GeV^2
We study the effects of instantons on the charged pion electro-magnetic
formfactor at intermediate momenta. In the Single Instanton Approximation
(SIA), we predict the pion formfactor in the kinematic region Q^2=2-15 GeV^2.
By developing the calculation in a mixed time-momentum representation, it is
possible to maximally reduce the model dependence and to calculate the
formfactor directly. We find the intriguing result that the SIA calculation
coincides with the vector dominance monopole form, up to surprisingly high
momentum transfer Q^2~10 GeV^2. This suggests that vector dominance for the
pion holds beyond low energy nuclear physics.Comment: 8 pages, 5 figures, minor revision
Twist-3 contribution to the amplitude in the Wandzura-Wilczek approximation
We have calculated the Wandzura-Wilczek contribution to the twist-3 part of
amplitude. It describes interaction of the
longitudinally polarized virtual photon with the real one, and it is suppressed
by 1/Q, where is the virtuality of the , as compared to the
twist-2 contribution. We have found that, in the Wandzura-Wilczek
approximation, factorization applies to the twist-3 amplitude.Comment: 13 pages, 4 figure
Unbiased analysis of CLEO data at NLO and pion distribution amplitude
We discuss different QCD approaches to calculate the form factor
F^{\gamma^*\gamma\pi}(Q^2) of the \gamma^*\gamma\to\pi^{0} transition giving
preference to the light-cone QCD sum rules (LCSR) approach as being the most
adequate. In this context we revise the previous analysis of the CLEO
experimental data on F^{\gamma^*\gamma\pi}(Q^{2}) by Schmedding and Yakovlev.
Special attention is paid to the sensitivity of the results to the (strong
radiative) \alpha_s-corrections and to the value of the twist-four coupling
\delta^2. We present a full analysis of the CLEO data at the NLO level of
LCSRs, focusing particular attention to the extraction of the relevant
parameters to determine the pion distribution amplitude, i.e., the Gegenbauer
coefficients a_2 and a_4. Our analysis confirms our previous results and also
the main findings of Schmedding and Yakovlev: both the asymptotic, as well as
the Chernyak--Zhitnitsky pion distribution amplitudes are completely excluded
by the CLEO data. A novelty of our approach is to use the CLEO data as a means
of determining the value of the QCD vacuum non-locality parameter \lambda^2_q =
/ =0.4 GeV^2, which specifies the average virtuality of
the vacuum quarks.Comment: 25 pages, 5 figures, 4 tables; format and margins corrected to fit
page size; small changes in the text and correction of misprint
Scheme dependence of NLO corrections to exclusive processes
We apply the so-called conformal subtraction scheme to predict perturbatively
exclusive processes beyond leading order. Taking into account evolution
effects, we study the scheme dependence for the photon-to-pion transition form
factor and the electromagnetic pion form factor at next-to-leading order for
different pion distribution amplitudes. Relying on the conformally covariant
operator product expansion and using the known higher order results for
polarized deep inelastic scattering, we are able to predict perturbative
corrections to the hard-scattering amplitude of the photon-to-pion transition
form factor beyond next-to-leading order in the conformal scheme restricted to
the conformal limit of the theory.Comment: RevTeX, 25 pages, 2 figures, 5 tables, minor changes, to be published
in Phys. Rev.
Hard exclusive photoproduction of mesons
We calculate the differential cross section and single-polarization
observables for the reaction within perturbative QCD,
treating the proton as a quark-diquark system. The phenomenological couplings
of gauge bosons to (spatially extended) diquarks and the quark-diquark
distribution amplitude of the proton are adopted from previous investigations
of baryon form factors and two-photon processes. Going beyond leading order, we
take into account hadron-mass effects by means of a systematic expansion in the
small parameter (hadron mass/ photon energy). With the -meson
distribution amplitude taken from the literature our predictions for the
differential cross section at | t | \agt 4 \text{GeV}^2 seem to provide a
reasonable extrapolation of the low-t data and are also comparable in magnitude
with the results of a two-gluon exchange model in which the gluons are
considered as a remnant of the pomeron. For momentum transfers of a few GeV
hadron-mass effects appear still to be sizeable.Comment: 37 pages, 7 figures, uses RevTeX styl
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